Start studying early, highers there's quite a lot of course content so if you're wanting to get a good grip on the course, the longer the better. Now, don't work yourself to burn out, but little and often is quite good.
If your school has Scholar, it's a very good resource for learning the material, also check online for booklets from other schools. Also read through the understanding standards to see model answers, and check the learning outcomes to know what the base knowledge you need. Both on the SQA website.
Hope it goes well for you!